Lamb Wave-based Non-destructive Evaluation in Thin Section Lap Joint of Friction Stir Welding

Weld integrity evaluation is essential for safety and reliability determination in various aerospace, petrochemical, civil as well as mechanical industries. Lamb wave-based Non-Destructive Evaluation (NDE) is a cutting-edge technology for Structural Health Monitoring (SHM) systems, offering the ability to monitor similar, dissimilar, as well as composite materials continuously. Also, suitability for in situ, autonomous applications, and possibility of inspecting non-accessible areas have proven Guided Waves (GW)-based NDE methods as one of the best contenders, specifically for thin section components. In this investigation, Lamb waves are used for weld quality prediction in terms of their process parameters. Friction Stir Welded (FSW) samples of 2 mm thickness and 245 mm × 180 mm size are joined in lap configuration with uniform overlap length. Piezoelectric wafer transducers are attached at a distance of 30 mm from the stir zone, and Laser Doppler Vibrometer (LDV) were implemented to analyze out of the plane displacement. A low voltage amplifier is used here to generate a 60 V signal, also damping tapes are incorporated at the sample boundary for noise reduction. For the thin section lap joint of Al5052, symmetric mode (A0) was used for weld characterization of friction stir welding parameters comprising tool tilt angle and overlap length using a single variable (at a time) approach. Although higher modes are also generated, therefore frequency-wavelength filtering techniques have been adopted for mode separation and subsequent signal processing. The operational signal used in this investigation comprises a modulated sinusoid with central frequency of 60 kHz. Preliminary study shows the frequency response dependency of tool tilt angle in signal propagation. As tool tilt angle increases, signal transmission increases up to a certain point due to better bonding capability, i.e., weld quality but after that it decreases due to the possibility of defect formation like wormholes, voids, and tunnel, etc. Sound weld formation is a result of balanced material mixing, i.e., optimum velocity of material flowing inside the stir zone, which results in adequate heat input and peak temperature. The developed prediction can be updated in future for accommodating nonlinear effects, dissimilar metal plates, etc.